1. Field of the Invention
The present invention relates to a motor control device, and more particularly, to a motor control device which decelerates a motor by limiting the torque of the motor at the time of a power failure of a power source.
2. Description of the Related Art
In a motor control device which controls a motor in a machine tool or an industrial machine, alternating current power at an alternating current power source side is converted to direct current power to output the direct current power to a DC link, and then the direct current power is further converted by an amplifier to alternating current power, which is supplied as drive power of the motor.
When a power source which supplies electric power for driving the motor has been stopped, it is preferable to stop the motor which drives a feed axis and the like in a machine tool as soon as possible.
For example, as disclosed in Japanese Patent No. 5612058, in a machine tool including a feed axis motor which drives a feed axis and a main axis motor which drives a main axis, there is known a method of avoiding an overvoltage alarm or a low voltage alarm by accelerating or decelerating the main axis motor in accordance with a value of a DC link voltage when a power failure occurs at an alternating current power source side.
Further for example, as disclosed in Japanese Patent No. 5746276, in a machine tool including a feed axis motor which drives a feed axis and a main axis motor which drives a main axis, there is known a method capable of securely stopping the feed axis motor at an early stage by performing command to output an excitation current larger than an excitation current commanded by a master control means to the main axis motor if an operation of the feed axis motor satisfies a predetermined determination condition even when a power failure occurs at an alternating current power source side, and of restraining heat generation of the main axis motor at the time of a normal operation.
Still further, for example, from Japanese Unexamined Patent Publication (Kokai) No. 2016-25828, there is known a motor control device that quickly stops a motor which drives a feed axis, while avoiding an overvoltage alarm after a power failure.
As methods of decelerating the motor, there are a method of generating a deceleration torque in the motor (hereinafter referred to as “deceleration by control”), and a method of applying a dynamic brake by connecting a resistance to the motor and allowing a current to flow therethrough, thereby allowing energy to be consumed (hereinafter referred to as “deceleration by hardware”). When a power failure occurs, power supply from the power source to the motor is interrupted, and thus the “deceleration by control” is performed using electric power as accumulated in the DC link. Since in general, a distance until the motor stops (braking distance) by the “deceleration by control” is shorter than that by the “deceleration by hardware”, to stop the motor because of a power failure as soon as possible, securing a long time for the “deceleration by control” within limited electric power is preferable.
Usually, in decelerating the motor, due to a regenerative operation, motive energy decreases and electric power (electric energy) increases, in stopping the motor when a power failure occurs as soon as possible, a time of the “deceleration by control” can be secured to be long within limited electric power.
However, depending on specifications of the motor, there occurs a situation in which when the deceleration torque is allowed to be too large in the “deceleration by control”, instead of the regeneration, powering that consumes electric power is performed, which consumes limited electric power. FIG. 5 is a circuit diagram illustrating an equivalent circuit of the motor per phase. Further, FIG. 6 is a diagram illustrating a relationship between a winding current of the motor and electric power. Assuming that the winding current of the motor is i [A], a terminal voltage of the motor is V [V], an angular velocity of the motor is ω (t) [rad/sec], a winding resistance of the motor per phase is R [Ω], an inductance per phase is L [H], and a counter electromotive force coefficient of the motor is Kv [V×sec/rad], an electric power P [W] of the motor is represented by expression 1.
                                                        P              =                            ⁢              Vi                                                                          =                            ⁢                                                Ri                  2                                +                                  Li                  ⁢                                      di                    dt                                                  -                                                      K                    V                                    ⁢                                      ω                    ⁡                                          (                      t                      )                                                        ⁢                  i                                                                                        (        1        )            
In expression 1, let the current be constant (di/dt=0), and rearrangement is made with respect to the winding current i of the motor, which becomes as expression 2.
                    P        =                                            R              ⁡                              (                                  i                  -                                                                                    K                        V                                            ⁢                      ω                                                              2                      ⁢                      R                                                                      )                                      2                    -                                                    K                V                2                            ⁢                              ω                2                                                    4              ⁢              R                                                          (        2        )            
As represented by expression 2, the electric power P of the motor is represented by a quadratic function of the winding current i of the motor, which is illustrated by a graph as FIG. 6. In FIG. 6, the horizontal axis represents the winding current i of the motor, and the vertical axis represents the electric power P of the motor. As apparent from FIG. 6, if the winding current i of the motor is less than KVω/R, regeneration of the electric power is performed due to deceleration of the motor, whereas if the winding current i of the motor is no less than KVω/R, consumption of the electric power is performed due to deceleration of the motor. Because the torque of the motor is proportional to the winding current of the motor, it is apparent from expression 2 and FIG. 6 that depending on specifications of the motor, when the deceleration torque is allowed to be too large in the “deceleration by control”, instead of regeneration, a powering operation which consumes the electric power is performed.
In stopping the motor when a power failure occurs as soon as possible, if the deceleration torque is allowed to be too large in the “deceleration by control” so that consumption of electric power is performed, and electric power in the DC link decreases, a device (common power source) which supplies the power source comes to fail to supply electric power, and the low voltage alarm is generated in an amplifier (inverter). When the low voltage alarm is generated, a switch from the “deceleration by control” to the “deceleration by hardware (dynamic brake)” is made. If such a switch is made early, a distance for the motor to stop (braking distance) becomes long.